This invention relates to ion exchange systems for removing arsenic from water. More particularly it relates to fixed bed ion exchange systems for removing arsenic which are configured to yield the flexibility and efficiency of moving bed systems.
Ion exchange is a chemical process often used to separate certain contaminant substances from a drinking water supply containing a mixture of several other harmless dissolved substances. For example, common ground water used for drinking water will contain substances such as the ionic forms of calcium, magnesium, sulfate, chloride and bicarbonate. In many cases, the water may also contain contaminants that are known to be detrimental to health. Such ionic substances as nitrite, nitrate, arsenic, antimony, fluoride, selenate, chromate, perchlorate and other similar harmful substances are often found. It is desirable to separate out the contaminants harmful to health by treating the water with an ion exchange system. There is a special interest in removing arsenic to very low levels.
Ion exchange processing systems range in production capacity from 50 gallons per day (GPD), such as is used in home water softeners and water purification devices, to very large plants having a capacity of several million gallons per day (50 to 100 million GPD) for centralized treatment of a public water supply.
Various equipment configurations or systems of vessels, plumbing and valves are used to apply the ion exchange process to the above purpose of treating a water supply to remove undesirable substances. For example, one prior art system is shown in FIG. 1 as system 100. This system is referred to as a single xe2x80x9cfixed bedxe2x80x9d design. The water to be treated is pumped from line 10 through a vessel 12 containing a bed 14 of ion exchange resin. Purified water is removed via line 16. Note that the word xe2x80x9csinglexe2x80x9d indicates that all process streams flow through the vessel 12 only once before continuing flow. Also the term xe2x80x9cfixed bedxe2x80x9d indicates that all ion exchange vessels are fixed in their positions.) During operation, there is no visible change in the positioning of the vessels or piping or any other component, only the internals of the valves change as they go from open to closed. (In contrast, when a moving bed system is in operation the position of the vessels and piping change and a multiport valve remains in a fixed position.
The vessel 12 containing the bed 14 is equipped with about eight to eleven different valves which control which process stream passes through the ion exchange bed. These are large full capacity valves capable of handling 50 to 100 percent of the peak flow rate through the plant. Practical flows of 500 to 1000 gallons per minute or more capacity for valve passage are not uncommon. By selecting the proper set of valves to be opened or closed either manually or by electronic controls, the flow of water to be treated by being passed through the vessel 12 and resin bed 14 can be stopped when the resin bed is exhausted. Control valve operations allow a sequence of process steps to be executed involving rinsing, regenerating and back washing and declassification (if required) to restore the adsorptive capacity of the resin. This sequence of steps produces a quantity of waste water that contains waste salt materials. This quantity of waste water is discarded. In FIG. 1 regenerant solution, such as brine, is shown supplied via line 18 and removed via line 20 and rinse liquid is shown being supplied via line 22 and removed via line 24.
Use of a single fixed bed of the prior art is also similar to a batch operation in that the flow of treated water is stopped completely while the resin goes through the resin regeneration steps. If an uninterrupted flow of treated water is desired, at least two fixed bed units must be used in parallel. Each bed is operated as above. After the first bed is exhausted, the bed is taken off line and regenerated while the second bed is placed into operation.
In general, a fixed bed system is comprised of as few vessels as is economically possible from the cost equipment point of view. Keeping the number of vessels to a minimum also reduces the number of large valves to be maintained or replaced. It also simplifies the valve control system with fewer valves to operate. It is customary therefore for plant designers to minimize the number of vessels to keep the number of valves to a minimum.
There are disadvantages, however, because larger vessels and large valves are required. To maintain or replace vessels or valves on a twelve foot diameter vessel, two or three men are required with the aid of heavy equipment lifting devices. Operation and maintenance costs will rise when first equipment costs are low because of large vessels. A popular design of a fixed bed system uses three vessels. Twenty four to thirty three large valves must be operated and maintained on such a system.
With a fixed bed system it is also often required to declassify the resin bed after regeneration. This step requires time and process water and produces additional waste water. The present invention eliminates this step.
Another prior art ion exchange system is known as a moving bed system or as a xe2x80x9cmerry-go-roundxe2x80x9d design. In this system the ion exchange resin is contained in several small vessels containing only an inlet port and an outlet port. Multiport valves communicate with these ports and control which process stream flows through each vessel. FIG. 2 depicts such a system as 200. These systems eliminate the use of large vessels and the subsequent high maintenance and replacement costs. In these systems multiple vessels 12, such as eighteen vessels numbered 1 through 18 are mounted on a circular platform 26 near the perimeter of a platform that slowly rotates while the system is in operation. The vessels 12 are each coupled through a line 32 to an upper multiport valve 28 and through a line 34 to lower multiport valve 30. Valves 28 and 30 can be combined or separate as shown.
The multiport valves are constructed with fixed (in and out) ports corresponding in position to the (in and out) ports of the ion exchange vessels which rotate part. The types of process streams flowing through the various vessels is controlled by the multiport valves 26 and 28 and is dependant on the position of the vessel on the circular platform. Consequently, as the platform rotates, the process stream entering and leaving any of the vessels changes according to a predetermined and difficult to alter process flow, set by the multiport valves.
Returning to FIG. 2, the system 200 shown therein has eighteen discreet vessels 12 and eighteen discreet positions for a vessel on the circular, rotating platform 26. The rotation of the platform physically moves each vessel from one position to the next position with all eighteen vessels moving simultaneously. The multiport valves 26 and 28 are positioned in the center of the rotating platform. The main process streams of treated water, regenerant, and rinse are first fed to the central multiport valves that then select the appropriate process stream for each position into which a vessel can be placed.
For example, a single vessel physically moves from position to position as shown in FIG. 2 When a given vessel is in positions 4 through 18 on the merry-go-round, it is fed untreated water from line 10 through valve 26 and line 30 which it purifies and discharge via line 32, valve 28 and line 16. As the vessel moves from position 4 through to 18 it continues in water treatment service but at each successive step the resin becomes more and more loaded with contaminant until it is virtually exhausted in position 18. When the vessel is moved into positions 1 through 2, a brine stream enters the vessel via line 22, valve 28 and line 32 to regenerate the resin by displacing contaminant off of it. Spent regenerant is removed via line 34, valve 30 and line 24. When the vessel is moved into position 3, a rinse and/or backwash stream enters the vessel via line 18, valve 30 and line 34 to displace regenerant solution. Rinse is removed via line 32, valve 28 and line 20. After making a complete rotation around the merry-go-round the vessel again enters the adsorption section starting at position 4 and advances step by step again to repeat the cycle.
One result of this configuration is the elimination of the large single port valves which were required for the fixed bed design. Practical designs for the moving bed systems incorporate numerous small vessels as dictated by mechanical stability and weight distribution considerations. The most mechanically stable systems use several (ten to forty) small vessels mounted on the xe2x80x9cmerry-go-roundxe2x80x9d to obtain an evenly distributed mechanical load.
These conventional systems present the following disadvantages.
High Wastewater Production
Conventional ion exchange systems are usually designed to keep equipment costs and operator and maintenance costs to a minimum while producing a water suitable for consumption. The generation and disposal of wastewater produced by ion exchange systems is usually a less important consideration. Conventional systems will produce from two to ten percent of the plant production as wastewater. The present invention minimizes waste water production and minimizes those operating costs dealing with the production and disposal of waste water. In many cases, the disposal of waste is a major cost of operation and becomes most important when operation over several years is considered. The invention produces as little as ten to thirty percent of the waste produced by conventional designs.
High Valve Maintenance and Spatial Requirements
Another disadvantage of the fixed bed system is the large number of heavy and bulky automatic valves needed to control the process flows through each vessel and the use of large diameter vessels. The main disadvantage of the moving bed system is that it requires two to three times the space and also requires very large and complex specialized multi port valves and a complex plumbing design. The net result is a far more costly systemxe2x80x94approximately three times the cost of its fixed bed counterpart.
Mechanical Instability and Cost
Another disadvantage of the moving bed system is its inherent mechanical instability. It presents a high center of gravity on top of a central mounting pivot. This design is subject to relatively small earthquake forces. Steel girder supports are often required to enhance stability, but cost increases.
Design Inflexibility
Disadvantages common to both systems of the art in comparison to the invention are that the process flow design for each conventional system must be fixed at design time. Fixed mechanical elements will determine the process stream that enters and leaves each vessel. To alter the process design at run time, the valves built into the rotating platform or the multiport valve, which rotates in unison with the rotating platform, must be mechanically altered or completely redesigned. Run time changes in a fixed bed system will also require physical changes to the system such as re-plumbing a portion or all of the vessels and valves.
The present invention allows flexibility in process design and equipment and optimum placement of vessels and piping to maximize process efficiency and minimize wastewater production. It permits any vessel to be out of service at any time. Other advantages are discussed below.
This invention provides a special water treatment system for removing arsenic comprised of a combination of ion exchange vessels, valves, piping and plumbing, electronic controls and processing sensors. This system is more efficient to construct, maintain and operate than conventional systems. The invention combines features of fixed bed systems with those of moving bed systems.
The invention applies to the treatment of water having typical drinking water components such as calcium, magnesium, sodium and chloride ions but also containing arsenic and optionally other undesirable inorganic contaminants such as nitrate, perchlorate, antimony, chromium, selenate and/or vanadium ions.
A particular advantage of the invention is its ability to provide treated water with a markedly reduced amount of waste water being produced.
We now have devised a fixed bed system for ion exchange removal of arsenic from water which embodies the advantages of a moving bed system without the size and cost of a moving bed design. The present design involves employing a substantial plurality (at least ten and preferably from about ten to about twenty-five) of fixed bed vessels which do not move but which can be accessed by the various process flows using a series of controller-actuatable valves, for example microprocessor-controlled valves. The system uses closely clustered, fixed position, multiple vessels combined with valves and piping so arranged to obtain the cost advantages of using small mass-produced vessels and valves, and a combination of easily maintained valves.
The present invention achieves (1) high process efficiency, (2) process flexibility, (3) low wastewater production, and (4) construction compactness and maintenance ease.
The invention uses several relatively small diameter fixed vessels each with two ports, one on each opposite end. These ports are closely associated with small volume headers. These headers are connected to manifolds used to conduct the process fluids to and from the vessels. A nest of small, easily-accessible process control valves is mounted between the headers and the manifolds.
Thus, in one aspect this invention is embodied as a system for continuously removing arsenic and other contaminants from arsenic contaminated water. This system includes a plurality of immobile vessels, each containing a resin bed capable of binding the contaminants from the contaminated water and yielding purified water and a contaminated resin bed. The vessels each have a first fluid communication opening (port) at a first end and a second fluid communication opening at a second end. The resin bed is located between the two ports.
Each vessel has two headers directly adjacent to the two ports. These headers are connected to the ports with a minimum of dead volume. Each of the headers is directly connected through automatically-actuatable valves to a series of manifolds which supply the various process feeds and accept the various process products.
The actuatable valves are controlled by a controller to flow arsenic-contaminated water from a manifold through the resin beds in a first subset of the plurality of vessels. This causes these resin beds to remove arsenic and other contaminants from the contaminated water and deposit the contaminants upon the resin in the beds and yield treated water. This treated water is removed from these vessels to a second manifold. The controller sets other valves to simultaneously flow regenerant solution from a manifold through at least one resin bed in a second subset of the plurality of vessels to regenerate its resin bed and to remove spent regenerant solution from these vessels. The controller also directs other valves to flow rinse water from a manifold through at least one regenerated resin bed in a third subset of the plurality of vessels to rinse its regenerated resin bed and to pass spent regenerant and/or used rinse water from the vessels in this third subset. The arsenic-loaded used regenerant is treated to recover arsenic
In another aspect this invention is embodied as a continuous process for purifying arsenic containing water. This process involves the following steps:
Contaminated water is fed through a first manifold to individually-valved first headers each directly adjacent to a first port of a first subset of a plurality of immobile vessels. Each of these vessels contains a resin bed between this first port and a second port. The resin bed is capable of binding arsenic from the contaminated water and yielding treated water and an arsenic-contaminated resin bed.
Treated water is removed through the second port from each of the vessels in the first subset, and passed through a second individually-valved header directly adjacent to the second port and through a second manifold to a treated water discharge.
Simultaneously, regenerant solution is fed to an individually-valved header directly adjacent to a first or second port on one or more additional vessels making up a second subset of the plurality. Each of the vessels in this second subset contains an arsenic-contaminated resin bed. The regenerant solution is passed over the contaminated resin bed so that the regenerant displaces the arsenic and other contaminants off of the contaminated resin bed to yield a regenerated resin bed and spent regenerant solution which is removed from the other port on the vessel and through another individually-valved header directly adjacent to this port. This regenerant solution is then processed to isolate the arsenic contaminant as a solid which is collected and handled as a toxic waste.
At the same time that the first subset of vessels is removing arsenic and producing purified water, rinse water is fed to an individually-valved header directly adjacent to a first or second port on one or more additional vessels making up a third subset of the plurality. Each of the vessels in this subset contains a resin bed that has been treated with regenerant. The rinse water is passed over the regenerated resin bed to yield a rinsed, regenerated resin bed and used rinse water which is removed from the other port on the vessel and through the individually-valved header directly adjacent to this opening.
In preferred embodiments, the directions of flow of the water regenerant and rinse are specified and the flows of regenerant and rinse are in series through more than one vessel.